1. Field of the Invention
The present invention generally relates to semiconductor devices, and particularly relates to a semiconductor device which operates in synchronism with different clock frequencies in different operation modes.
2. Description of the Related Art
In semiconductor integrated circuits, it is desirable to use high operation frequencies to achieve high-speed operations. At the same time, there is a need to reduce power consumption in the circuits. It is difficult, however, for the same circuit to satisfy demands for high-speed operations and reduced power consumption at the same time. In general, circuits capable of high-speed operations consume large power, whereas circuits of low power consumption can operate only at low operation frequencies.
Semiconductor integrated circuits capable of operating in synchronism with high-speed clock signals can, of course, operate in a low-speed mode using low-frequency clock signals. Since such semiconductor integrated circuits have circuit configurations specifically designed for high operation frequencies, however, these circuits in the low-speed mode consume electric power as large as that consumed in high-speed operations.
To achieve the above two demands, a circuit for high-speed operations and a circuit for low power consumption may be implemented on the same semiconductor chip, so that these circuits can be switched between high-speed operations and low-speed operations with the circuit for low power consumption being used in the low-speed operations. In this case, however, a check has to be made as to which one of the high-speed mode or the low-speed mode is used, in order to switch these circuits. Unless a register or the like for storing mode settings is provided, the check has to be made based on a clock signal input to the chip.
Accordingly, there is a need for a semiconductor device which switches internal circuits in order to cope with both high-speed operations and low-power-consumption operations by checking a current operation mode based on an input clock signal.